Numbers catalogs Israel's journey from the foot of Mount Sinai to the land of Canaan (the Promised Land). Along the way the Israelites are whining and complaining about their present circumstance. Because of their lack of faith it takes them 40 years to complete a journey that should have only taken no longer than 3 months.
"Not one of those who saw my glory and the signs I performed in Egypt and in the wilderness but who but who disobeyed me and tested me ten times--not one of them will ever see the land I promised on oath to their ancestors. No one who has treated me with contempt will ever see it." Numbers 14:22-23
The 40 years that pass mean that an entire generation will die on the journey before the Israelites reach the Promised Land. All those who grumbled and did not have faith in the Lord were punished by ending their days in the desert.
However, it is important to see God's patience and grace in this story. He preserves his people and still delievers them into the Promised Land in the end.
Even during the lesson He is teaching them, God blesses His people:
"May the Lord bless you and protect you. May the Lord smile on you and be gracious to you. may the Lord show you his favor and give you him peace." Numbers 6:24-26
The most important lesson Numbers teaches us is to trust God and live by faith.